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United States Patent |
5,167,732
|
Naik
|
December 1, 1992
|
Nickel aluminide base single crystal alloys
Abstract
Nickel aluminide single crystal alloys having improved strength and
ductility at elevated temperatures, produced by major elemental additions
to strengthen the Ni.sub.3 Al phase by solid solutioning and/or secondary
phase formation. The major elemental additions comprise (by weight) 7-20%
Al, 0.5-9% molybedenum, 0.5-10% tungsten and 2-15% titanium. Optional
minor elemental additions of boron, manganese, silcon and/or hafnium are
preferred.
Inventors:
|
Naik; Subhash K. (Milford, CT)
|
Assignee:
|
Textron, Inc. (Providence, RI)
|
Appl. No.:
|
770631 |
Filed:
|
October 3, 1991 |
Current U.S. Class: |
148/404; 420/460 |
Intern'l Class: |
C22C 019/03 |
Field of Search: |
148/409,404,410,428,429
420/460
|
References Cited
U.S. Patent Documents
4801513 | Jan., 1989 | Duhl et al. | 148/404.
|
Primary Examiner: Dean; R.
Assistant Examiner: Phipps; M.
Attorney, Agent or Firm: Perman & Green
Claims
What is claimed is:
1. A nickel aluminide single crystal alloy composition having excellent
stress rupture strength and oxidation resistance over a broad temperature
range consisting essentially by weight:
about 7.0% to about 20.0% aluminum;
about 0.5% to about 9.0% molybdenum;
about 0.5% to about 10.0% tungsten;
about 2.0% to about 15.0% titanium;
about 0.0% to about 0.2% boron;
about 0.0% to about 0.5% manganese;
about 0.0% to about 0.5% silicon;
about 0.0% to about 0.5% hafnium; and
the balance nickel.
2. An alloy composition according to claim 1 consisting essentially of by
weight:
about 7.0% to about 15.0% aluminum;
about 1.0% to about 8.0% molybdenum;
about 1.0% to about 8.0% tungsten;
about 3.0% to about 8.0% titanium;
about 0.0% to about 0.1% boron;
about 0.0% to about 0.05% manganese;
about 0.0% to about 0.15% silicon;
about 0.0% to about 0.2% hafnium; and
the balance nickel.
3. An alloy composition according to claim 1 consisting essentially of by
weight:
about 8.0% to about 12.0% aluminum;
about 5.0% to about 7.0% molybdenum;
about 5.0% to about 7.0% tungsten;
about 4.0% to about 6.0% titanium, and
the balance nickel.
4. An article of manufacture comprising material fabricated from the
composition of claim 1.
5. An article of manufacture comprising material fabricated from the
composition of claim 3.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improved nickel aluminide single crystal
base alloy compositions having superior tensile strength and
stress-rupture strength and capable of being wrought or cast into shape by
single crystal casting technology at a high or standard solidification
rate.
Single crystal nickel aluminide alloys of different compositions are well
known as proposed substitutes for single crystal nickel chromium alloys,
or stainless steels, in the event that chromium becomes unavailable.
Nickel aluminide can be cast as single crystal Ni.sub.3 Al, or can exist as
polycrystalline nickel aluminide. The Ni.sub.3 Al phase is brittle and
drops in strength above about 1400.degree. F. The ductility of Ni.sub.3 Al
has been improved by the minor addition of boron. However, greater
improvements in strength and ductibility at elevated temperatures, up to
about 1600.degree. F., are necessary to permit the use of modified
Ni.sub.3 Al alloys for higher temperature applications.
2. Description of the Prior Art
It has been proposed to alter the properties of nickel aluminide alloys by
the addition thereto of various ingredients.
U.S. Pat. No. 4,677,035 discloses high strength nickel base single crystal
alloy compositions having high stress-rupture strength at elevated
temperatures, such as 1800.degree. F./20 ksi for 1000 hours. Such
compositions contain relatively high amounts of chromium and cobalt, have
unsatisfactory stress rupture strength at low temperatures and have
unsatisfactory oxidation resistance and corrosion resistance.
U.S. Pat. No. 4,885,216 discloses improved nickel base alloy compositions
having similar high temperature stress-rupture strength properties as the
alloys of U.S. Pat. No. 4,677,035 but having improved oxidation resistance
and corrosion resistance due to the incorporation of small amounts of
hafnium and/or silicon and optional small amounts of yttrium, lanthanum
and/or manganese. However the alloys of this Patent also have
unsatisfactory stress-rupture strength at low temperatures
U.S. Pat. No. 4,612,164 discloses the inclusion of boron, hafnium and/or
zirconium in nickel aluminide alloys to improve ductility and yield
strength up to about 133 ksi at elevated temperatures up to about
850.degree. C. (1562.degree. F). The addition of titanium, molybdenum
and/or tungsten is not suggested.
U.S. Pat. No. 4,711,761 issued on an application referred to in U.S. Pat.
No. 4,612,165, and discloses Ni.sub.3 Al alloys to which manganese,
niobium and titanium are added to improve fabricability. The nickel
aluminide alloys are doped with boron and a substantial weight of iron,
but the amount of titanium is only 0.5 weight percent. Such
iron-containing compositions have limited tensile strength and temperature
capabilities.
U.S. Pat. No. 4,478,791 discloses the addition of boron to nickel aluminide
alloys to improve the strength and ductility thereof, and U.S. Pat. No.
4,613,489 discloses that the loss of ductility of such cast composition
during annealing can be avoided by subjecting them to hot isostatic
pressing. Compositions containing specific amounts of titanium, molybdenum
and/or tungsten are not disclosed.
U.S. Pat. No. 3,933,483 discloses the addition of at least 10% by weight
molybdenum and up to 2.5% by weight of silicon to nickel aluminides in
order to increase the tensile strength at elevated temperatures and the
toughness at room temperatures without impairing the oxidation-resistance
thereof. The addition of tungsten and/or titanium is not disclosed, and
silicon is a melting point depressant.
Related U.S. Pat. No. 3,904,403 further discloses the addition of titanium,
chromium, zirconium, niobium, tantalum or tungsten to silicon-containing
nickel aluminide alloys. No compositions containing molybdenum, tungsten
and titanium are disclosed.
Other prior art patents of interest include U.S. Pat. No. 4,461,751 and
2,542,962.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(c) shows the DTA curve of a preferred alloy ISC-5 of the present
invention as compared to the DTA curves for control base alloys ISC-1,
ISC-3 and ISC-6 shown in FIGS. 1(a), 1 (b), and 1 (d) respectively;
FIG. 2 illustrates the relative yield strengths, over various temperatures,
of the present alloy ISC-5 as compared to control base alloys;
SUMMARY OF THE INVENTION
The object of this invention is to provide a modified nickel aluminide base
single crystal intermetallic alloy of superior tensile strength and
stress-rupture strength, at temperatures ranging between room temperature
up to about 1600.degree. F. and good corrosion resistance and oxidation
resistance. The present alloys can be wrought or cast into useful shapes,
as for gas turbine engine components. The present alloys may be easily
cast in an equiaxed form, or may be cast at standard or high
solidification rates in single crystal form for particular utility as
power turbine blades in a gas turbine engine.
According to the embodiments of the present invention, fibers or whiskers
or fabrics thereof can be incorporated into the present alloys to form a
metal matrix composite, further enhancing suitability for fabricating
highly stressed rotating components such as turbine blades.
The foregoing objects, and others, are accomplished by providing a novel
nickel aluminide based alloy composition comprising by weight about:
______________________________________
BROAD MORE MOST
RANGE PREFERRED PREFERRED
______________________________________
aluminum 7.0%-20.0% 7.0-15% 8.0-12.0%
molybdenum
0.5%-9.0% 1.0-8.0% 5.0-7.0%
tungsten 0.5%-10.0% 1.0-8.0% 5.0-7.0%
titanium 2.0%-15.0% 3.0-8.0% 4.0-6.0%
boron 0%-0.2% 0-0.1% --
manganese
0%-0.5% 0-0.05% --
silicon 0%-0.5% 0-0.15% --
hafnium 0%-0.5% 0-0.2% --
bal. nickel
bal. nickel bal. nickel
______________________________________
Currently, turbine blades capable of operating at the highest temperatures
are cast in single crystal form. Compared to polycrystalline material, the
elimination of grain boundaries enhances creep resistance, a primary
requirement for high temperature turbine blades. The alloys heretofore
known and commonly used for casting into single crystal blades have been
primarily nickel base. In the heretofore known alloys, the ductile gamma
phase is strengthened by dispersing throughout it a harder, more brittle
gamma prime phase, the tri-nickel aluminide (Ni.sub.3 Al).
On the binary nickel-aluminum system phase diagram, the tri-nickel
aluminide is denoted as the gamma prime phase, and is found to occur in a
small range of aluminum contents between 23.0 and 27.5 atomic percent, or
13.6 and 14.0 weight percent.
With the matrix of the known control alloys based on the gamma prime phase,
the ultimate strength of such alloys is limited by the weakness of the
gamma prime phase. The approach in the current invention is to employ a
matrix of predominantly trinickel aluminide, which heretofore has suffered
from poor ductility and low strength, and to improve its properties
through solid solution and/or additional phases being present. This
disadvantage has been lessened to some extent, according to U.S. Pat. Nos.
4,612,165 and 4,711,761, by minor additions of other elements such as
iron, boron or manganese. According to the present invention, the solid
solution strength of the base matrix is substantially increased by
additions of molybdenum, titanium and tungsten. Furthermore in the
investigation of alloys encompassed by this invention, the effect of
replacing aluminum with titanium was determined. Trinickel aluminide and
metastable trinickel titaniumide produce an isomorphus structure in the
compositions of the present invention.
The following compositions were prepared in the evaluation of the present
invention, as listed in Table I below. Eight of the compositions were
formed into single crystal test specimens. Listed in Tables 2 and 3 are
the density, x-ray diffraction results and the incipient melting
temperatures as determined for these latter eight compositions.
TABLE 1
______________________________________
NOMINAL COMPOSITIONS (WT %) OF CANDIDATE
INTER-METALLIC SINGLE CRYSTAL (ISC) ALLOYS
Alloy
Designation
Composition
______________________________________
ISC-1 Ni--14Al (control)
ISC-2 Ni--12.8AL--6.8Mo--6.8W
ISC-3 Ni--13.8Al--6.8Mo--6.8W
ISC-4 Ni--7.2Al--10.2Ti--6.8Mo--6.8W
ISC-5 Ni--10.2Al--5.2Ti--6.8Mo--6.8W
ISC-6 Ni--14Al--0.1B (control)
ISC-7 Ni--12.8Al--6.8Mo--6.8W--0.1B
ISC-8 Ni--13.8Al--6.8Mo--6.8W--0.1B
ISC-9 Ni--7.2Al--10.2Ti--6.8Mo--6.8W--0.1B
ISC-10 Ni--10.2Al--5.2Ti--6.8Mo--6.8W--0.1B
______________________________________
TABLE 2
______________________________________
DENSITY AND X-RAY ANALYSIS OF ISC-X ALLOYS
Density
Alloy (lb./in..sup.3)
XRD Analysis
______________________________________
ISC-1 0.268 Ni.sub.3 Al, NiAl (control)
ISC-2 0.283 Ni.sub.3 Al, W(Mo)
ISC-3 0.280 Ni.sub.3 Al, NiAl, W(Mo)
ISC-4 0.287 Ni.sub.3 Al, NiAl, W(Mo), Ni.sub.3 Ti
ISC-5 0.288 Ni.sub.3 Al, NiAl, W(Mo)
ISC-6 0.266 Ni.sub.3 Al, NiAl (control)
ISC-8 0.284 Ni.sub.3 Al, NiAl, W(Mo), W.sub.2 B
ISC-10 0.286 Ni.sub.3 Al, NiAl, W(Mo), W.sub.2 B
______________________________________
TABLE 3
______________________________________
DTA SUMMARY OF ISC-X ALLOYS
Incipient Melt Temperature
Alloy (.degree.F.)
______________________________________
ISC-1 (control)
2505
ISC-2 2409
ISC-3 2427
ISC-4 2328
ISC-5 2386
ISC-6 (control)
2438
______________________________________
The x-ray diffraction analysis indicates that the alloys consist of two to
four phases. Comparing alloys No. ISC-2 and -3, the slightly higher
aluminum content of alloy No. ISC-3 results in the presence of the NiAl
phase. Interestingly, a titanium content of 5.8% as in alloy No. ISC-5
does not result in the presence of the Ni.sub.3 Ti phase which appears in
alloy No. ISC-4 which has a higher titanium content. The boron additions
of 0.1% in alloys No. ISC-6 through 10 were much larger than the 100 to
400 ppm by weight used by Oak Ridge National Laboratories (ORNL Baseline
in FIG. 2). The larger additions of boron were to investigate the effects
of larger boron content on ductility. It was also believed that the low
levels of boron would increase production cost in that more exact control
would be required. However, the inclusion of boron in alloy NO ISC-6, in
the absence of molybdenum and tungsten, was found to reduce the
stress-rupture or yield strength to unacceptable levels at room
temperature, as shown in Table 4.
The object is to develop compositions which exhibit higher tensile strength
capability (from RT to 1600.degree. F.) over known Ni.sub.3 Al alloy
compositions.
Table 1 lists the alloy designations along with their nominal compositions.
Briefly, ISC-1 is the known baseline alloy and ISC-2 to ISC-5 are alloys
with major additions of Mo and W, with and without Ti. The intent was
twofold: (1) identify the solid solubility limit of W and Mo in the
Ni.sub.3 Al phase in an effort to strengthen the phase through solid
solutioning and/or secondary phase formation; and (2) determine the
effects of substituting Ti for Al in the ordered NiAl phase. Alloys ISC-6
to -10 are similar compositions as -1 to -5; however, 0.1 percent B was
added to verify if ductility could be improved.
As shown by Table 2, the density of the baseline Ni.sub.3 Al (ISC-1) is
0.268 lb/in..sup.3 while densities for modified chemistry alloys (ISC 2-5)
range from 0.280 to 0.288 lb/cu in. Since the density of nickel base
single crystal alloys produced according to our aforementioned U.S. Pat.
No. 4,677,035 is 0.312, it can be concluded that the present intermetallic
single crystal alloys have 8 to 16 percent lower density than the prior
known nickel base single crystal alloys. XRD analysis indicates that the
candidate alloys consist of two to four phases. Comparison of XRD results
for ISC-2 and -3 indicate that for the same W, and Mo content, the higher
Al content (13.8 2t% A, ISC-3) results in the NiAl phase A lower Al
content (i.e., 12.2 to 12.8 wt% Al) if only the Ni.sub.3 Al phase is
desired. A titanium content of 5.8 wt. % does not result in Ni.sub.3 Ti
phase (e.g. see ISC-5) while larger Ti contents (10.2 wt. % in ISC-4)
result in a separate Ni.sub.3 Ti phase. The boron additions (0.1%) in
ISC-6 to -10 were much larger than those used by ORNL (100 to 400 ppm).
This was done to verify the effects of large boron contents on ductility.
It was also felt that low levels of boron would in turn increase alloy
procurement cost, due to the stricter controls required during production.
Therefore, the intent was to identify the upper limits of boron required
for improved ductility while easing the specification requirements. The
XRD analysis indicated that 0.1 wt. % B would form the W.sub.2 B phase.
DTA studies were conducted to determine the melt temperature of the tested
alloys. FIG. 1 shows typical DTA curves of alloys ISC -1, -3, -5 and -6.
Table 3 lists the incipient melt temperatures of ISC-1 to -6 alloys. The
baseline or control alloy (ISC-1) indicated the highest incipient melt
temperature of about 2505.degree. F. The incipient melt temperature of the
modified composition alloys ranged from 2386.degree. F. to 2427.degree. F.
while the other control composition, ISC-6, had the second highest melt
temperature of 2438.degree. F. Titanium addition has a severe effect on
lowering incipient melt temperatures (>120.degree. F.). Also, as expected,
the addition of 0.1B lowers the incipient melt temperatures of ISC-1 by
about 65.degree. F.
Based on DTA studies, alloys were solution heat treated to verify if any
solutioning or change in microstructure could potentially occur. There was
more ordered dendritic type phase distribution after heat treatment. The
strength properties in the as-cast and heat treated condition alloys were
determined to evaluate performance. Table 4 summarizes the tensile results
(UTS, Y.S. Elongation, R/A) of various alloys ISC 1-3, -5, -6 and -8 from
RT to 1600.degree. F. The tensile strength peaks around 1100.degree. F.,
as expected. It should be noted that ISC-1 alloy corresponds very closely
to the ORNL developed NI.sub.3 Al alloy. Comparing data between various
alloys, it is clear that alloy ISC-5 shows superior tensile, elongation
and R/A properties at both room temperature and elevated temperatures.
Alloy ISC-5 exhibits a remarkable 60 percent improvement in strength over
the baseline Ni.sub.3 Al alloy ISC-1 at all temperatures.
TABLE 4
______________________________________
SUMMARY OF TENSILE DATA FOR ISC-X ALLOYS
Temp. UTS YS Elong. R/A
Alloy (.degree.F.)
(ksi) (ksi) (%) (%)
______________________________________
ISC-1 RT 63,700 44,300 11.6
1100 97,200 76,400 4.9 10.9
1400 85,100 85,100 2.3 4.4
1600 55,600 53,800
ISC-2 RT 87,450 71,100 1.5 4.4
1600 60,800 54,000 4.1 6.9
ISC-3 RT 73,200 61,900 0.7 3.0
1100 124,400 101,300 3.9 8.0
1400 83,800 74,800 8.1 14.3
1600 48,900 38,400 15.2 22.3
ISC-5 RT 117,600 96,200 1.0 4.4
1100 135,200 120,700 1.3 5.1
1400 119,450 114,600 0.9 4.4
1600 93,300 88,700 5.5 10.1
ISC-6 RT 70,600 37,000 3.3 14.3
1100 131,900 122,000 6.6 13.0
1400 121,600 -- 1.1 3.0
1600 109,400 109,400 3.5 5.9
ISC-8 RT 99,500 81,500 1.1 4.4
1100 125,400 106,300 2.2 5.9
1400 90,100 80,100 7.8 10.2
1600 57,000 49,300 9.8 16.4
______________________________________
FIG. 2 shows the relative performance in yield strengths from RT 31
1600.degree. F. between the present ISC-5 alloy and an advanced alloy
(U.S. Pat. No. 4,711,761) developed by ORNL/NASA. The ORNL/NASA alloy is
based on Ni.sub.3 Al +FE +Dopants. The baseline alloys (ISC-6 and
NI.sub.3 AI +0.05% B, also shown in U.S. Pat. No. 4,711,761) have also
been included for reference. ISC-5 has 11% higher strength than the best
alloy of U.S. Pat. No. 4,711,761.
The results of the S-R testing of the 3 alloys whioh showed the most
potential for engine application (for e.g., power turbine blades) are
given in Table 5. All alloys exhibited greater than 1000 hour life at
1100.degree. F./65 ksi. However, at higher temperature (e.g., 1200.degree.
F./44 ksi), ISC-5 was clearly superior.
TABLE 5
______________________________________
STRESS RUPTURE SUMMARY OF NI.sub.3 AL
MODIFIED ISC ALLOYS
Sample Temp. Stress Life Elong. RA
Ident. (.degree.F.)
(ksi) (hrs) (%) (%)
______________________________________
ISC-3 1100 65 1075.5
10.6 7.3
ISC-5 1100 65 1007 Retired
Retired
ISC-8 1100 65 1437 7.5 13.5
ISC-3 1200 55 75 7.8 6.5
ISC-5 1200 55 1008 Retired
Retired
ISC-8 1200 55 135 -- 6.5
ISC-5 1500 25 123 31.5 25
______________________________________
The microstructural stability of ISC-5 was considered as excellent, both
the as-cast microstructure and the microstructures of ISC-5 S-R tested at
1100.degree. F, 1200.degree. F. and 1500.degree. F. for long time
exposures. The oxidation resistance of ISC-5 was superior with no evidence
of oxidation attack even on exposures to 1500.degree. F S-R tested bars of
ISC-5 evidence excellent oxidation resistance (no oxide layer). Thus the
present invention provides Ni.sub.3 Al modified SC alloys which show
superior performance over prior known Ni.sub.3 Al type alloys.
Currently, a high emphasis is placed on light weight, high specific
strength titanium aluminide alloys. To date, (.alpha.-2 Ti.sub.3 Al
(Ti-25Al-13Nb 1 Mo) and .alpha.-TiAl (Ti-40Al-lV) with temperature
potential of 1100.degree. F. and 1500.degree. F. respectively, have been
identified for compressor (for e.g., impeller) and power turbine (for e.g.
blades) applications.
ISC-5 has the capability of exceeding the performance of both of these
titanium aluminide alloys. Typically the densities of .alpha.-3 Ti.sub.3
Al and .alpha.-TiAl are 0.17 and 0.14 lbs/cu-in respectively, while ISC-5
has a density of 0.27 lbs/cu-in. The comparative S-R life at 1200.degree.
F./55ksi for .alpha.-2 Ti.sub.3 Al and ISC-5, respectively, is 300 hours
compared to greater than 1007 hours. It is apparent that ISC-5 has a
greater than 2.11X improvement over alpha-2 on a density corrected basis.
The comparative yield strength of .alpha.-TiAl and ISC-5 on a density
corrected basis (normalized to TiAl) shows that ISC-5 represents a greater
than 30 percent improvement at 1500.degree. F. over .alpha.-TiAl. Also,
based on comparing available literature data (AFWAL-TR-82-4086), ISC-5
exhibits an improvement of over 10 percent in S-R life at 1500.degree. F.
when normalized to .alpha.-TiAl density.
Therefore, ISC-5 alloy is excellent for application in power turbine blades
or other light-weight structural component applications. ISC-5 is easily
castable to net shape, whereas TiAl has major problems with casting due to
its brittleness and cracking problems. Additionally, the as-cast
properties of ISC-5 are significantly superior over the complex (e.g.,
Isoforge +HIP +heat treatment) processed .alpha.-TiAl. Reduced processing
leads to greater cost savings for components fabricated from the ISC-5
alloy.
Preferably the present single crystal alloys are produced as composites
containing temperature resistant fibers whiskers or fabrics, such as
infiltrated fabrics of single crystal alumina available under the
trademark Saphikon. The selection of suitable fibers, whiskers and/or
fabrics will be apparent to those skilled in the art in the light of the
present disclosure, as will be the processes for producing such
composites, such as by investment casting in the withdrawal process.
It is to be understood that the above described embodiments of the
invention are illustrative only and that modifications throughout may
occur to those skilled in the art. Accordingly, this invention is not to
be regarded as limited to the embodiments disclosed herein but is to be
limited as defined by the appended claims.
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